fix(fleet): rename Traffic tab label to Routing for consistency (#1119)

The Fleet view sub-tab that renders RoutingTab.tsx was labeled "Traffic"
while every adjacent identifier already used "Routing": the backend
route file (backend/src/routes/mesh.ts), the component path, the
localStorage key (sencho-routing-view-mode), the SegmentedControl aria
label ("Routing view mode"), and the engineering vocabulary across the
codebase. Operators looking for the "Routing tab" could not find it
because the visible label said something else.

Align the user-visible label with the rest of the implementation and
update the two doc references that named the tab by its old label
(docs/features/fleet-actions.mdx, .env.example).

.env.example

@@ -106,7 +106,7 @@ SSO_CALLBACK_URL=
 
 # Experimental UI surfaces. When unset or any value other than "true",
 # the UI hides surfaces that are kept in the repo but not yet promoted
-# to the default 1.0 build (Fleet Traffic · Routing tab, Fleet
+# to the default 1.0 build (Fleet Routing tab, Fleet
 # Deployments / Blueprints tab, Fleet Federation tab, Fleet Secrets
 # sync tab, Fleet Actions tab). Backend routes for these surfaces stay
 # live regardless. This flag controls UI discovery only.